Maugan Ra is a surefire inclusion, as are the Dark Reapers as if it was Maugan Ra's home craftworld then there would have been a large number of them.
That would be your heavy and a HQ choice taken, tho I would consider some other heavy unit choice. Maybe war walkers, they have an apocalypse formation call the shadow walker formation, in this it suggests that the Eldar can fit up war walkers with cloaking technology - which would aid in avoiding and ambushing chaos attacks.
I think you need to assume that Altansar has used stealth and suprise tactics to survive as a lone craftworld would find it hard to fight off the onslaught of chaos. Spirit engines such as wraithguard and wraithlords would be highly vulnerable to loosing the Eldar spirits, I don't think that they would risk this.
Also rangers leave the craftworld, so again, not really an option - unless you are saying they are "like rangers" having spent the time in the eye of terror learning evasive skills - this I like the idea of.
Harlequins are your obvious choice for "fluffy" elites, if you are going unique and fluffy I would take a full compliment of them, they must have aided Altansar in its survival.
For fast attack, maybe Vypers, though not hugely effective they do fit the fluff. They are smaller than tanks and in theory manoeuvrable. Fit them with BL and have them jump around popping things, or SC/SC and use them to clear troops.
So the army runs as follows:
Rangers as troops hide on objectives and represent the hidden and ready to ambush Altansar population.
Dark reapers lead by Maugan Ra stand stoicly ready to pound the enemy.
War walkers lurk (if you can get your opponent to agree to it then try using them with the rules from the shadow walker formation, tbh you are playing such a sub optimal list to get a nice fluff feel I am sure most opponents would let you)
Harlequins do their thing (as a side note, I would model these up to all have tradegy masks from theatre in tribute to the tragedy of the craftworld)
Vypers whip about mowing down units.
It would make a nice fluffy army and would have a rather odd and unique play style, it really depends on what you are trying to do.